on the 6.0, there is a release button on the inside part of the tensioner. pull the belt up until you feel the tensioner give upwards, then you slide the release button to the side to hold the belt tensioner in place.
i had to do this in morrow, ga in the parking lot of allan vigil ford under a light pole on top of a hot engine about 8:00 pm one night. its a little hard but can be done. sorry if my explanation is not very good. if you need to call me, just PM me and i might be able to explain it better. oh yeah, the dealership gave me a diagram that shows where the tensioner release was. you might able to get this diagram from your local ford dealer.